Molecules of this family have essentially been isolated from the vegetable kingdom, principally in the reproductive system of plants, whereas they appear to be absent from the green parts of plants (Martin-Tanguy et al., Phytochemistry, 17: 1927-1928, 1978; FEBS Lett., 108: 176-178, 1979). In 2003, Alemano et al. (Annals of Botany, 92: 613-623) identified N-caffeoyl-tyrosine and N-caffeoyl-DOPA in the somatic embryogenesis cells of Theobroma cacao. Later, in grains of green coffee (robusta), p-coumaroyl-N-tyrosine, feruloyl-N-tyrosine, feruloyl-N-tryptophan and caffeoyl-N-phenylalanine were identified (Clifford and Knight, Food chemistry, 87: 457-453, 2003), whereas their antioxidant capacity had already been found previously (Sanbongi et al., J. Agric. Food Chem. 46: 454-457, 1998). Then, Stark and Hoffman (J. Agric. Food Chem., 53: 5419-5428, 2005) and then Stark et al. (J. Agric. Food Chem., 54: 2859-2867, 2006) identified and completed the list with other analogs present in the same vegetable sources.
The amides of cinnamic and hydroxycinnamic acids have in particular been listed and evaluated for their important antioxidant activities (Spasova et al., J. Peptide Sci., 12: 369-375, 2006), a property that seems inherent to any structure of this type.
Other molecules such as 4-hydroxycinnamoyl-(L-phenylalanine methyl ester) amide or 3,4-dihydroxycinnamoyl-(L-aspartic acid dibenzyl ester) amide have been cited for their antioxidant activity against oxidation of LDLs (low-density lipoproteins) by copper, and at the same time these molecules inhibit the activity of human acyl-CoA:cholesterol acyltransferase (Lee et al., Biorg. Med. Chem. Letters, 14: 4677-4681, 2004); they are therefore implicated in hypercholesterolemia. The N-cinnamoyl amides have also been described as antiplatelet agents (Hung et al., Bioorganic and Medicinal Chemistry, 13: 1791-1797, 2005). The clovamides (N-cinnamoyl tyrosine), which are phytoalexins that accumulate in plants following an attack by pathogens (Yamamoto et al., Pharmacol Biochem Behav, 40: 465-469, 1991), also belong to this group, and in this connection are known to be powerful antimicrobials.
Finally, vaso-relaxing activities of N-caffeoylamino R acid derivatives (R=GABA or hexanoic acid) have also been described recently (Iizuka et al., Yakugaku Zasshi, 123: 963-971, 2003).
The cinnamoyl amide derivatives of amino acids therefore constitute a family of molecules with considerable potential both in the therapeutic and in the cosmetic field, or as preservatives. However, their isolation from plants is difficult, the yields and the amounts are low, and the structures have little diversification.
Methods for the synthesis of certain of these molecules by the chemical route have been described; but they require several stages, use toxic compounds and in consequence are difficult to industrialize:
In the synthesis described by Iizuka et al. (Yakugaku Zasshi, 123: 963-971, 2003), four stages are necessary: (1) protection of the hydroxyl functions of the ring by ethyl chloro-acetate in a basic medium, (2) then coupling by dicyclohexylcarbodiimide (DCCDI); (3) the amino acid is then coupled in the presence of triethylamine, and finally (4) a hydrolysis liberates the hydroxyl functions from the aromatic nucleus (deprotection). N-caffeoylglycine, N-cinnamoylglycine, N-caffeoyl-β-alanine, and N-cinnamoyl-β-alanine were synthesized by this method.
Another quicker synthesis is based on fixation of the amino acid (L-aspartic acid or L-tryptophan in this case) on the corresponding acyl chloride after prior protection of the hydroxyl groups of the aromatic ring by dimethylaminopyridine (DMAP). The amidation reaction is carried out by reflux in the presence of tetrahydrofuran (THF), followed finally by deprotection to liberate the hydroxyls (Stark and Hofmann, J. Agric. Food Chem., 53: 5419-5428, 2005; Stark et al., J. Agric. Food Chem., 54: 2859-2867, 2006).
We may also cite the works of Tada et al. (J. Oleo Sci., 51: 19-27, 2002), which describe the synthesis of various derivatives including the methyl ester of N-caffeoyl-O-acetylserine which has strong antioxidant and tyrosinase-inhibiting activity. Moreover, Lee et al. (Bioorg Med. Chem. Lett., 14: 4677-4681, 2004) carried out the synthesis of esters of p-coumaroyl amide of aspartic acid (methyl ester) and of phenylalanine (dibenzyl ester) respectively, by a method equivalent to Iizuka et al. (Yakugaku Zasshi, 123: 963-971, 2003) (see above).
There was therefore still a need for a simple method, easy to apply, industrializable, permitting the preparation of cinnamoyl amide derivatives of amino acids.